Gas sensors are used in a variety of applications which require qualitative and quantitative analysis of gases. In the automotive industry, it is well known that the oxygen concentration in the exhaust gas of an engine has a direct relationship to the air-to-fuel ratio of the fuel mixture which is supplied to the engine. As a result, oxygen gas sensors are used in automotive internal combustion control systems to provide accurate oxygen concentration measurements of automobile exhaust gases for determination of optimum combustion conditions, maximization of fuel economy, and management of exhaust emissions. To be useful, oxygen sensors should have a rapid response time at temperatures ranging between about -40.degree. C. and 800.degree. C.
As illustrated in U.S. Pat. No. 3,844,920 to Burgett et al., the electrochemical type of oxygen sensor typically used in automotive applications utilizes a thimble-shaped electrochemical galvanic cell operating in the potentiometric mode to determine, or sense, the relative amounts of oxygen present in an automobile engine's exhaust. This type of oxygen sensor includes an ionically conductive solid electrolyte material, typically yttria stabilized zirconia, a porous electrode coating on the sensor's exterior which is exposed to the exhaust gases, and a porous electrode coating on the sensor's interior which is exposed to a known concentration of reference gas. The individual components are typically fabricated by conventional processes such as molding, grinding and high temperature firing.
The gas concentration gradient across the solid electrolyte produces a galvanic potential which is related to the differential of the partial pressures of the gas at the two electrodes according to the Nernst equation: EQU E=AT 1n(P.sub.1 /P.sub.2)
where E is the galvanic voltage, T is the absolute temperature of the gas, P.sub.1 /P.sub.2 is the ratio of the oxygen partial pressures of the reference gas at the two electrodes, and A=R/4F, where R is the universal gas constant and F is the Faraday constant.
Currently, these potentiometric oxygen sensors are employed in the exhaust gas system of an internal combustion engine to determine qualitatively whether the engine is operating at either of two conditions: (1) a fuel-rich or (2) a fuel-lean condition, as compared to stoichiometry. After equilibration, the exhaust gases created by engines operating at these two operating conditions have two widely different oxygen partial pressures. This information is provided to an air-to-fuel ratio control system which attempts to provide an average stoichiometric air-to-fuel ratio between these two extreme conditions. At the air-to-fuel stoichiometric point, the oxygen concentration changes by several orders of magnitude. Accordingly, potentiometric oxygen sensors are able to qualitatively indicate whether the engine is operating in the fuel-rich or fuel-lean condition, without providing more specific information as to what is the actually air-to-fuel ratio.
Current oxygen sensors which operate in the aforementioned potentiometric mode are sufficiently sensitive to operate satisfactorily about the fstoichiometric point for purposes of indicating whether the engine is operating in the fuel-rich or fuel-lean condition. However, because their output voltage is a function of the natural log of the oxygen partial pressure ratio, a potentiometric sensor does not produce an output that is useful for determining the air-to-fuel ratio at operating conditions away from the stoichiometric point.
Due to increasing demands for improved fuel utilization and emissions control, more recent emphasis has been on wide range oxygen sensors capable of accurately determining the oxygen partial pressure in exhaust gas for internal combustion engines operating under both fuel-rich and fuel-lean conditions. Such conditions require an oxygen sensor which is capable of rapid response to changes in oxygen partial pressure by several orders of magnitude, while also having sufficient sensitivity to accurately determine the oxygen partial pressure in both the fuel-rich and fuel-lean conditions. The output of a potentiometric oxygen sensor does not provide sufficient resolution to quantify small changes in exhaust gas oxygen partial pressures when operating away from the stoichiometric point, and therefore cannot accurately determine the air-to-fuel ratio under fuel-rich or fuel-lean conditions.
The prior art has suggested that oxygen sensors which produce an output proportional to the air-to-fuel ratio may offer significant performance advantages for future engine control systems. As taught by U.S. Pat. No. 4,863,584 to Kojima et al., U.S. Pat. No. 4,839,018 to Yamada et al., U.S. Pat. No. 4,570,479 to Sakurai et al., and U.S. Pat. No. 4,272,329 to Hetrick et al., an oxygen sensor which operates in the diffusion limited current mode produces such a proportional output which provides sufficient resolution to determine the air-to-fuel ratio under fuel-rich or fuel-lean conditions. Generally, diffusion limited current oxygen sensors have a pumping cell and an oxygen storage cell for generating an internal oxygen reference. A constant electromotive force is maintained between the storage cell and the pumping cell so that the magnitude and polarity of the pumping current can be detected as being indicative of the exhaust gas composition.
While the above diffusion limited current oxygen sensors generally provide satisfactory performance, there remains the need to further improve the construction of these devices to reduce their costs and make them more readily producible under mass production conditions. Specifically, each of the oxygen sensors taught by the above prior art relies upon structure which forms a chamber or gap, features which complicate processing and assembly in mass production.
Thus, it would be desirable to provide an oxygen sensor for an internal combustion engine operating within both the fuel-rich and fuel-lean conditions which is extremely sensitive and capable of rapid, precise, and absolute oxygen concentration measurements, while also being amenable to mass production.